Early review of std.logger

[Dicebot]

As `std.logger` is still marked as "work in progress" this thread 
is less formal that typical pre-voting review. Goal is to provide 
as much input about desirable `std.logger` functionality and 
current state and let module author to use this information for 
preparing proposal for actual review / voting.

Lets unleash the forces of constructive destruction.

===== Meta =====

Code: 
https://github.com/D-Programming-Language/phobos/pull/1500/files
Docs: 
http://burner.github.io/phobos/phobos-prerelease/std_logger.html

First announcement / discussion thread : 
http://forum.dlang.org/post/mailman.313.1377180809.1719.digitalmars-d@puremagic.com